Burn-Rate Modifiers

In addition to fuel, oxidizer and inert/energetic plasticizer, various additives are also included in the propellant formulations in order to improve their mechanical or ballistic properties. One such additive is termed a burn-rate (ballistic) modifier which alters inherently high dependence of the burn rate on chamber pressure. The use of an additive for this purpose depends on the type of propellant, that is, 

composite or composite modified double-base (CMDB). Tables 4.10—4.14 depict various BRMs employed for this purpose. 

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Uses Addn of 1 mole % of anhyd Cu perchlorate lowers the decompn point of K perchlorate from 618 to 525° (Ref 15). Addn of 0.015 mole % to AP lowers the induction period for its decompn at 233° from 27 4 to 6 2 mins (Ref 17). Cu perchlorate has been patented as a burning rate modifier in solid propints (Ref 12) Refs l)Gmelin, System No 60, PartB-1 (1958), 337 2) G.S. Serullas, AnnChimPhys (2) 46,... 

Bfx, Fx, and related compoimds are the subject of a great munber of invention patents especially related to its uses in material sciences. For example, they were included in the formiflation as rubber additives [245,246], as inhibitors in the polymerization of aromatic vinyl monomers [247], as components in the igniting composition for inflation of airbags [248,249], as explosives [250-253], as soHd propellants [254], as burn-rate modifiers [255], and as liquid-crystalHne materials [256]. 

Though the major components of double-base propellants are NG-NG or NG-TMETN, various additives such as plasticizers, burning rate modifiers, or combustion instability suppressants are needed. Table 4.11 shows the materials used to formulate double-base propellants. 

The specific impulse of each pyrolant is computed as a function of air-to-fuel ratio, as shown in Fig. 15.7. In the computations, the pressure in the ramburner is assumed to be 0.6 MPa at Mach number 2.0for a sea-level flight When GAP pyrolant is used as a gas-generating pyrolant, the specific impulse is approximately 800 s at e = 10. It is evident that AP pyrolant and NP pyrolant are not favorable for use as gas-generating pyrolants in VFDR. However, the specific impulse and burning rate characteristics of these pyrolants are further improved by the addition of energetic materials and burning rate modifiers. 

Fig. 15.8 shows a typical set of burning rate versus pressure plots for GAP pyrolants composed of GAP copolymer with and without burning-rate modifiers. The burning rate decreases as the mass fraction of the burning-rate modifier, denoted by (G), is increased. Graphite particles of diameter 0.03 pm are used as the burn-... 

Fig. 15.8 Burning rate characteristics of a GAP copolymer with and without burning-rate modifier (graphite), showing that the pressure exponent increases with increasing amount of burning-rate modifier.

Pyrodex" is a patented pyrotechnic composition designed to fulfill many of the funetions of blaek powder. It eontains the three ingredients found in blaek powder plus binders and burning rate modifiers that make the material somewhat less sensitive and slower burning. A greater degree of confinement is required to obtain performance comparable to "normal" black powder [6]. 

Basically, PVC plastisol propellants are systems of binder,.oxidizer, and metallic fuel (if used). Minor ingredients, normally comprising less than 2% of the total, consist of wetting agent, stabilizer, opacifier, and burning rate modifier. The composition of three typical PVC plastisol propellants is shown in Table I. 

Burning rate catalyst Burning rate negative catalyst Negative burning rate modifier Metal fuel... 

These propellants are also called colloidal or double-base (DB) propellants and have nitrocellulose (NC) and nitroglycerine (NG) as their principal ingredients. Some ingredients may also be added in small proportions to serve as stabilizer, non-explosive plasticizer, coolant, lubricant, opacifier and burning-rate modifier or other additives to impart desirable properties to the propellants. These propellants are processed into desired shapes by two methods, that is, extrusion and casting. They find wide applications in anti-tank missiles. 

Table 4.10 Burn-rate modifiers for double-base rocket propellants.

Table 4. Less toxic or non-toxic burn-rate modifiers for DB rocket propellants.

Ultrafine carbon fibers Non-toxic burn-rate modifiers for cast DB propellants in the absence of lead salts [248]. 

Table 4.14 Burn-rate modifiers for composite propellants. ...

Less Toxic and Non-Toxic Burn-Rate Modifiers for DB Propellants... 

Burn-rate modifiers probably affect most of these combustion steps, that is, the endothermic and exothermic reactions and heat losses. Rastogi et al. have shown that burn rate, surface temperature, flame temperature and rate of decomposition are enhanced in case of catalyzed propellants while these are lowered in case of burn-rate retarders. This may be due to heat produced in catalytic reactions in the former case whereas bum rates are reduced on account of endothermicity of the condensed phase reactions on the propellant surface in the case of retarders. It is also reported that carbonates of copper and chromium are better catalysts... 

Camp, A.T., and Crescenzo, F.G. (1964) Copper and lead burning-rate modifiers for DB propellants containing aluminium. US Patent 3,138,499. 

Rb and Cs salts of bitetrazole amines (BTAs) as burning rate modifiers for pyrotechnic formulations based on CsN03 and Ba02 together with fuels such as and Si and VAAR as non-sooting binder [141]. 

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